Refactor redis and queue dependencies in preparation for Django 4.0

[Kobzol]

After this PR, it should be hopefully straightforward to migrate to Django 4.0 (will send in a follow-up PR).

We need to perform special operations on production to make it work with this PR!

Ideally by stopping the server, performing the migration steps manually, and then restarting it and let the after-merge deploy be a no-op.

What needs to be done:

• Reconfigure RQ worker to use the --with-scheduler flag for the default queue: rqworker default --with-scheduler
• Disable/remove the rqscheduler service
• Configure SCHEDULER_QUEUES in local_settings.py (needs to be done with admin access)
• Run the following commands to skip already applied scheduler migrations:

  $ python3 manage.py migrate scheduler 0001_initial_squashed_0005_added_result_ttl --fake